This invention pertains to the production of superplastically formed, complex, metal alloy structures, and more particularly to these structures having curved surfaces.
Superplasticity is the characteristic demonstrated by certain metals that develop unusually high tensile elongation with a minimum necking when deformed within a limited temperature and strain rate range. This characteristic, peculiar to certain metal and metal alloys has been well known in the art. It is also well known that at these same superplastic forming temperatures, some materials will fusion bond with the application of pressure at the contacting surfaces.
U.S. Pat. Nos. 4,217,397 and 4,304,821 to Hayase et al and assigned to the same assignee as the instant case teaches the process for making a sandwich structure in which metal work sheets are joined in a preselected pattern by an intermittent weld. The joined sheets are sealed by a continuous weld to form an expandable envelope. The application of inert gas pressure to the envelope in a fixture superplastically produces the sandwich structure. Core configuration of the structure is determined by the intermittent weld pattern. The face sheets of the sandwich structure may be formed from one sheet of the envelope or may be inserted in the limiting fixture and the envelope expanded against the face sheets. The contents of U.S. Pat. No. 4,217,397 and 4,304,821 are incorporated herein by reference.
Basically, the process as taught in these two patents is limited to producing a core structure which is flat, i.e., the face sheets are flat and not curved. Although they suggest preforming the face sheets for complex shapes, as a practical matter, this technique is effective only wit very limited curvature. The most difficult and complex part of this procedure is welding the preformed core sheets. In addition to the welding, preforming is an added complex operation because it requires precision forming or the welding cannot be satisfactorily performed. In the typical four-sheet process a different die radius is required for each of the two face sheets and a third radius is required for the welded work sheets forming the envelope which is expanded to produce the core.
U.S. Pat. No. 4,113,522 to Hamilton et al and U.S. Pat. No. 3,340,101, to D. S. Fields, Jr., et al and an article which appeared in Steel magazine of Dec. 15, 1962 entitled "Superplasticity Enchants Metallurgy," by Professor Walter A. Backofen of Massachusetts Institute of Technology, all teach some type of two-step operation. They include means for at least Partially deforming the material by direct displacement (rather than through a fluid interface) and a second phase through a fluid interface which may occur before or after the direct displacement. However, all of these references teach superplastic forming of single sheets. Furthermore, all of these references teach retention of the sheet being formed by clamps. Professor Backofen shows ten ways to form superplastically. Although he identifies the retention means as a clamp, he illustrates it as a stop, which is believed to be intended as a schematic representation of a clamp because in certain methods, e.g., with the billow plug, billow snapback, air slip, and plug assist and air slip, if it were a stop alone, as illustrated, it would not work. No discussion of these methods is contained in the article.
There are a couple of other points which are significant by way of background. First, it is important to note that the secret to all superplastic forming is to keep the part being formed in tension, as any compression results in buckling and consequent wrinkles in the final part. Second, many alloys are being developed, particularly aluminum alloys, which demonstrate superplastic characteristics, but do not readily diffusion bond. Basically, all that has been taught in superplastic forming in combination with diffusion bonding applies to the more difficult alloys to bond, except that some subsequent alternate step must be taken to perfect the bonding, such as welding, brazing or bonding, all of which are known in the art.
Further, by way of background, typically when forming the multi-sheet envelope by fluid pressure, the material being formed is retained in the forming fixture by the hydraulically actuated portion of the press, which acts as a huge clamp, generally acting through a split forming die. However, when you are superplastically forming metal partially by direct displacement phase, and partially by a fluid interface, the hydraulically actuated portion of the press is required for the direct displacement phase, and some other means must be devised to retain the sheets being formed during the fluid interface phase. Double acting presses can be adapted to perform both functions, however, these presses are complex and expensive and are generally not readily available. It is highly desirable that in a single acting press, both the direct displacement and fluid interface forming are to be performed in one shaping die without removing a partially formed part between these steps. Hence, some other means must be devised to retain the sheets being formed during both forming operations.
It is an object of this invention to produce a curved sandwich structure by creep forming face sheets and/or the envelope to be expanded by direct displacement and further expanding some or all of the elements by a fluid interface.
It is a further object of this invention to provide means for retaining or holding sheets to be formed into the sandwich structure other than the press itself.
It is yet a further object of this invention to provide means within the means for retaining the sheets to be formed to provide excess material for the forming operation so as to minimize the thinning in the high-strained areas or control material thicknesses.
Another object of this invention is to perform the entire forming process in one forming fixture without a need for removing a partially formed structure for intermittent steps.